This invention relates to labels, more particularly, the invention relates to laminated labels comprising a cast face sheet layer and a pressure-sensitive adhesive layer. In one aspect, the invention relates to tamper-evident labels. In another aspect, the invention relates to labels suitable for high temperature applications such as those found in printed circuit board processing and automotive industries, while, in a further aspect, the invention relates to labels designed to dissipate static electricity that may be harmful to electronic components or clean rooms.
Labels for electronic components preferably meet a number of requirements. For example, the labels should not damage the electronic components with static electricity, should be able to withstand the heat and chemicals used in processing the electronic components and should be tamper-evident to prevent counterfeiting of the components.
Static dissipation is important for electronic components which are vulnerable to damage from very low voltage (e.g. 50 V) discharges. For example, computer board assemblies contain many static sensitive integrated circuit chips which bear barcode labels that are used for tracking and identification of the boards. These labels are potential sources of static electricity.
Static electricity is generated during application and removal of a label by a phenomenon known as triboelectric charging. Whenever two surfaces in close proximity are displaced, a charge imbalance is generated on each of the surfaces. On electrically insulative surfaces these charges are not dissipated and could build to an eventual rapid discharge (which can appear as a spark). These discharges can destroy the gate oxide layers inside of an integrated circuit chip, thus rendering it useless. Even low voltage discharges which do not generate a visible spark can destroy a modern integrated circuit.
Many current labels used on electronic components are not electrostatic dissipative labels. Many actions on the labels, such as printing on the top of the labels, separating the labels from the liner, application of the labels to a substrate, removal or repositioning of the labels on a substrate, or even the processes in which the labels are manufactured, can generate electrostatic charges on the label, liner and any surface it has been in contact with. These charges may be dissipated and damage the electronic circuitry on which the labels are placed and/or exceed the limits for static safety at work stations where circuit boards are assembled.
Current labels that are used on electronic components are typically four layers: top coat, face sheet, pressure-sensitive adhesive and release liner. The facesheet or backing layer material for high heat resistivity is usually expensive, such as polyethylene naphthalate (PEN) and polyimide face sheets. Most of the materials from which the label is built are generally electrically insulative in nature. Static electricity is generated at the time the label is peeled from the liner before application to the electronic part, and these charges can exceed hundreds of volts. During the peeling and placement operations, a danger exists that these charges will rapidly discharge and damage the circuitry in the vicinity at which the label is applied. The repositioning or removal of the label is another triboelectric charging event that also carries the danger of damage from rapid discharge. To reduce damage caused by electrostatic discharge, ANSI/ESD S20.20 page 7 is an industry specification that indicates a proposed maximum work surface potential of 200 Volts and use of packaging materials having surface resistivity greater than 105 ohms/square and less than 1012 ohms/square.
In addition, label stocks with the above construction can be removed and reused by counterfeiters. Label removal and switching is becoming a bigger problem. For brand name protection, reduction of fraudulent warranty claims and product liability reasons manufacturers are looking for ways to identify their own products and indicate that their product is not out of warranty. In some embodiments of this invention the label stock is made tamper-evident due to damage and destruction of the label that would result from attempted removal.
Labels for electronic circuit application may also be required to resist elevated temperatures from the soldering or solder reflow operation. For surface mount soldering the reflow temperature can be 180° C. to 220° C., while for through-hole soldering, a temperature exposure of about 300° C. is typical. In the future as lead is being replaced in solder alloys even higher temperatures may be required.
A current heat resistant label stock, 3M ScotchMark™ 3921 with an acrylic based cast face sheet, does not provide enough tamper-evident property on certain substrates and the label stock turns to yellow or brown at a relatively low temperature. Lintec™ heat resistant label stock HR2240-50 with an acrylic face sheet as well, does not provide good solvent resistance, and therefore is unsuitable for use in the printed wiring assembly processes that include a number of different cleaning procedures with harsh cleaners.
Some current labels with electrostatic dissipative properties, Brady B-477, Brady B-473, Brady B-478, and Brady B-479 are described in U.S. Pat. No. 5,958,537. The labels of U.S. Pat. No. 5,958,537 are comprised of a polyester or polyimide face sheet coated with an electrostatic dissipative primer layer, which in turn is laminated to an electrostatic dissipative pressure-sensitive adhesive layer. The primer layer and adhesive layer contain electrically conductive particles, e.g. metals. However, these electrostatic dissipative labels still have the potential to generate static electric charges, attract dust on the topside of labels and the liners which labels are carried on, and dissipate the charges to electronic components or printed circuit boards because the face sheet, top coat and liners are not static safe materials.
Therefore, the industry still needs a fully electrostatic dissipative label that has heat and solvent resistant properties and may also be tamper-evident.